This invention relates to electroluminescent polymers, electroluminescent devices containing these polymers, processes for generating electromagnetic radiation using these polymers, and processes for the preparation of these polymers.
In recent years, a great deal of research has been conducted into electroluminescent materials, that is to say materials which emit electromagnetic radiation (typically visible light) when an electric current flows through the polymer. Electroluminescent materials are potentially useful for the construction of image display devices, which could be very thin and lightweight, and could thus advantageously replace cathode ray tubes, gas plasma displays, liquid crystal displays and other types of image display devices.
Several different types of electroluminescent materials are known; see, generally as to development of such materials, International Patent Application No. PCT/GB90/00584 (Publication No. WO 90/13 148). The first type to be developed was inorganic semiconductor materials such as gallium phosphide and zinc sulfide. However, such inorganic electroluminescent materials are not readily usable in large image display devices, and many of them suffer from practical drawbacks, including poor reliability. Accordingly, most recent research has concentrated on organic electroluminescent materials.
Many organic compounds, especially polycyclic arenes such as anthracene, perylene, pyrene and coronene, are electroluminescent. However, electroluminescent devices using these monomeric organic compounds suffer from poor reliability, and also present difficulties in preparing the thin layers of the materials needed for use in practical electroluminescent image display devices, and the electrodes needed for electrical contact with such layers. Techniques such as sublimation of the organic material produce layers which are soft, prone to recrystallization and unable to support high temperature deposition of electrode layers, while techniques such as Langmuir-Blodgett film deposition produce films of poor quality, dilution of the active material and high cost of fabrication. Prior art electroluminescent devices formed from these materials, such as that described in U.S. Pat. No. 3,621,321, typically suffer from high power consumption and low light output.
Attempts have also been made to use solid solutions of monomeric organic electroluminescent materials in non-electroluminescent polymers as the active layer in electroluminescent devices; see, for example, U.S. Pat. No. 4,356,429. However, use of such solid solutions carries a substantial risk of phase separation by crystallization of the monomer out of the polymer, especially in environments where the electroluminescent device may be subjected to large changes in temperature. In addition, often it is difficult to find a non-electroluminescent polymer which can both conduct electricity and dissolve a large proportion of the active electroluminescent material to form the necessary solid solution. Finally, the use of a solid solution necessarily involves substantial dilution of the active electroluminescent material and thus lowers the maximum light flux from a given area of the electroluminescent device.
Accordingly, research has been carried out on electroluminescent polymers having an electroluminescent group incorporated into the polymer itself, particularly polymers containing a chain of electroluminescent groups (such as p-phenylene vinylene units) conjugated with one another along the chain; see, for example, International Patent Applications PCT/GB90/00584, PCT/GB91/01420 and PCT/GB91/01421 (Publication Nos. WO 90/13148, WO 92/03490 and WO 92/03491 respectively). These polymers do not suffer from the phase separation, solubility and stability problems encountered with the aforementioned solid solutions, and permit some control over the wavelength of the light emitted by variations in the chemistry of the electroluminescent polymer; for example, the aforementioned International Application PCT/GB91/01420 describes electroluminescent polymers comprising at least two different monomer units having different semiconductor bandgaps to control the conjugation length within the polymer, and hence the quantum efficiency and wavelength of light emitted. However, since in this type of polymer the electroluminescent groups are present in the main chain of the polymer, any desired modification of the physical characteristics of the polymer, or of its electroluminescent properties, often results in an undesirable change in the other. For ease of manufacture of electroluminescent devices, it would often be convenient to have a type of electroluminescent polymer in which substantial variations in the physical characteristics could be made without significant changes in its electroluminescent properties.
One polymer, namely poly(vinylcarbazole), is known having a repeating unit comprising an electroluminescent group directly attached to a non-conjugated main chain fragment; see the papers by R. H. Partridge in Polymer, 24, 733-762 (1983). According to these papers, poly(vinylcarbazole) itself is essentially non-electroluminescent, but the polymer may be rendered electroluminescent by mixing it with either an antimony pentahalide or an electroluminescent monomer, such as perylene or triphenylbutadiene. A very low work function alkali metal electrode is also required Such electroluminescent mixtures are potentially subject to the same type of phase separation and stability problems as the solid solution electroluminescent mixtures discussed above, and the use of the alkali metal electrode raises additional problems.
Accordingly, there remains a need for an electroluminescent polymer which permits variation in the physical characteristics of the polymer without substantial changes in its electroluminescent properties. In addition, the electroluminescent polymer should not require the presence of additives which pose stability or phase separation problems. Finally, it is desirable that the electroluminescent polymer have good electroluminescent efficiency (which is measured as quanta emitted per electron injected); the electroluminescent efficiencies of prior art polymers are typically of the order of 0.1%, so that large amounts of electrical energy are required to produce an electroluminescent image display device which can be viewed in normal room lighting, as is essential if, for example, the image display device is to be used as a computer monitor or television screen.
It has now been found that electroluminescent polymers having desirable properties can be formed by attaching, to a main chain essentially free from electroluminescent groups, a plurality of side chains, each of which comprises an electroluminescent group and a flexible spacer connecting the electroluminescent group to the main chain.